In a hard disk drive (HDD), herein also referred to as a file, data tracks are initially defined for the disks by the servo using a process called servo track writing (STW). Servo control is the precise positioning of the heads with respect to the data tracks in order to minimize whatever external disturbance, e.g., repeatable run-out (RRO), non-repeatable run-out (NRRO), misregistration, shock, vibration, etc., that might occur. Designers strive to minimize the impact of these disturbances. When the head moves off-track, they try to bring it back as soon as possible. However, if it gets too far off-track, there is a danger of writing over data on an adjacent track and destroying or erasing the existing data. Therefore, there is a value defined that is called the write inhibit threshold (WIT) value.
The write inhibit threshold value has an effect of limiting overall servo performance. The write inhibit threshold value is a number that is typically represented in servo units or as a percentage of track pitch. It is the amount of off-track a head is allowed to go during a writing operation that is deemed safe. That is to say, it is the amount that is determined to be allowed before damage could occur to the data on neighboring tracks. The write inhibit threshold value is like a safety valve which only exists to prevent the head from writing an unsafe distance off-track.
The writing off-track toward an adjacent (from track center) is sometimes referred to as a “squeeze” effect. Heads can have very wide distributions in their ability to handle squeeze effects. Some heads can tolerate more adjacent track squeezing from the writing process before errors are created on adjacent tracks. Other heads are a lot more sensitive to the squeeze and adjacent track errors can be created with a small squeeze effect. While the write inhibit threshold value is a needed requirement for data protection from off-track writing, it can impact file servo performance. The value used for the write inhibit threshold value is typically based on the results from the worst performing head in the population of heads that is to be used in a file product. That value is generally tighter than is necessary for the general population of heads.
Prior Art FIGS. 1A–1C are illustrations of data tracks showing variations in heads for writing data to the data tracks and the impact of the variations on write inhibit threshold values. Prior Art FIG. 1A shows a nominal sized head 105a residing above data track 110 that has a track mid-point defined by dotted line 120. Left adjacent data track 130 and right adjacent data track 140 are the tracks that would be susceptible to data errors or erasure should head 105a write too far off of track center 120. It is, however, possible that head 105a might be able to write off-track by an amount indicated by the length of arrow 160 toward left adjacent data track 130, or by an amount indicated by the length of arrow 150 toward right adjacent data track 140 without impacting either track.
Referring to Prior Art FIG. 1B, head 105b is a larger head than 105a of FIG. 1A. If head 105b were to write off-track by the amount indicated by the length of arrow 150 or the length of arrow 160, it might impact the data on left adjacent data track 130 or right adjacent data track 140.
In Prior Art FIG. 1C, a head 105c is shown that is smaller than either head 105a or 105b. Head 105c might be able to write off-track by an amount that is greater than either amount 150 or amount 160 without impacting either adjacent data track 130 or adjacent data track 140.
In the examples shown in FIGS. 1A, 1B and 1C, if heads 105a, 105b and 105c were to constitute the entire population of heads to be used in a file product, the write inhibit threshold value would be determined based on head 105b. Although size was used to exemplify differences in heads that might affect off-track performance, it is only one of several aspects of heads that can affect off-track performance. However, it can be seen from this example that, by basing the write inhibit threshold value on the off-track performance of head 105b, the write inhibit threshold value is tighter than it needs to be for heads 105a and 105c. The tighter number impacts the file's servo and mechanical systems since it generally takes longer to certify a tighter write inhibit threshold value before writing can take place. It also means that a writing process may be aborted sooner than needed. Also, revisions may be burned unnecessarily and/or unnecessary retries may be required if, during the writing process, it is found that the write inhibit threshold value was exceeded before the completion of writing all the data to the disk. This can have the observed effect of taking longer to perform writing operations, which impacts file performance.